Failure Assessment Diagram Research Articles

New formulation for fracture toughness characterization using four-point bend specimens

The present work aims to present a new complete procedure to characterize fracture toughness in terms of stress intensity factor K or the J−integral for Mode I of loading using four-point bend specimens. Four-Point Bend (4PB) specimens are shown to be valid configurations for fracture toughness characterization tests with certain advantages when compared to traditional three-point bend geometries and single edge notch tension SE(T) specimens under clamped conditions. Fracture toughness is a key parameter in defining critical flaw sizes in a fitness-for-service (FFS) procedure based upon fracture mechanics principles. Accurate estimation of fracture toughness is therefore necessary to assess the integrity of critical engineering structures by following the failure assessment diagram concept. Extensive 3D finite element analyses were performed to obtain the analytical relationships necessary to calculate the fracture toughness parameters. New formulas are provided for the elastic compliance, stress intensity factor, and eta factor for calculating fracture toughness valid for four-point bend specimens. Using the distance between inner and outer roller supports as a normalization parameter, the stress intensity factor and the normalized elastic compliance equations are shown to be independent of the specimen supporting configuration. The eta factor based upon the CMOD is dependent on the distance between external and internal roller supports as well as a∕W. Experimental fracture tests were carried out to check the practicality of the methodology. J-R curves obtained by four-point bend specimens were compared to resistance curves derived from three-point SE(B) specimens and clamped SE(T) samples which are standardized specimens. The results show that the four-point bend specimens provide reliable fracture toughness values somewhat higher than three-point bend geometries.

Structural integrity assessment of a 5CT P110 steel riser pipe according to BS 7910:2013 standard

In this work we performed a structural integrity assessment of a riser pipe made of a material with a novel application, the API 5CT P110 steel, with superficial semi-elliptical cracks in the longitudinal position. The behavior of different crack size on a pipe under specific boundary conditions and pressure loads was studied. With the Finite Element Method, the Stress Intensity Factors along the crack front were obtained and compared with the Stress Intensity Factors calculate by the analytical approach of BS 7910:2013 standard. Afterwards, the fracture toughness of API 5CT P110 steel was obtained through CTOD tests to plot a specific Failure Assessment Diagram according the Option 1 of BS 7910:2013, in order to determine the acceptability of the cracks. The critical crack depths in the studied conditions were 14.2 mm according to the analytical approach of BS 7910:2013 standard and 15.5 mm according to FEM analysis.

Analysis of Crack Behaviour in Pipeline System Using FAD Diagram Based on Numerical Simulation under XFEM

For a long time, cracked structures have triggered various researchers to develop a structural integrity approach and design models to address the fracture problems. In the present study, a pipeline with an axial semi-elliptical surface defect was examined in detail. Recent works have highlighted the use of the classical finite element method (CFEM) as numerical tools to solve the fracture mechanics; however, this approach comes with a few difficulties in the modelling aspects. To overcome this issue, we proposed the use of the extended finite element method (XFEM), which was implemented in the commercial version of Abaqus software. Moreover, we have used the results based on this technique in the volumetric method to estimate the stress intensity factors (SIFs). Then, this parameter was employed to build the failure assessment diagram (FAD). The FAD curve was used in the current investigation because it is one of the conventional methods for the evaluation of flaws in steel pipes. The XFEM simulations enable us to draw an FAD curve that can be used as a practical reference for defect evaluation in pipeline systems in the industrial world.

Computational Fracture Mechanics: Evaluation of The Structural Integrity in a Penstock Applying the BS7910 Standard and Finite Element Analysis

The present work aims to determine the structural integrity of penstocks, applying the BS7910 standard and finite element analysis (FEA). For the study of fracture mechanics in thin-walled cylinders that have defects inside, the structural integrity of the element is determined through a failure assessment diagram (FAD) where the fracture failure index (Kr) is graphed versus plastic collapse index (Lr). Based on the locus of the initial point of failure, the safety factor is calculated, and it is defined if the element is fit for service or if corrective action must be taken to continue operating; later, crack growth is analyzed, where the critical point of rupture of the penstock and therefore its useful life is determined. To contrast results, a simulation of fracture mechanics is performed in ANSYS, where the stress intensity factor (KI) is determined using the fracture tool, and the useful life of the element is also calculated through a fatigue analysis. The use of tetrahedral elements is recommended for the overall meshing and a cobweb configuration for meshing at crack-tip. Finally, the results obtained are compared, where the mean average percentage error of 3.24% was obtained, denoting the usefulness of the two methods as well as the simplicity of the Paris’ law.

The simplified approach for estimating probabilistic safety factors in fracture mechanics

Basic principles of the simplified probabilistic approach in structural integrity assessment are proposed to estimate probabilistic safety factors against fracture. The coefficients of variation for the material resistance are assumed to be the same independently on modes of failure and the resistance effects, namely, fracture or plastic collapse. The load for engineering components under consideration is assumed to be described by the stochastic distribution independently on modes of failure. In this case, probabilistic safety factor against collapse can be employed to estimate safety factors against fracture taking into account correlation of the safety factor against collapse in terms of the yield stress with safety factors against fracture in terms of the fracture toughness or the failure assessment diagram. Thus, safety factors against fracture calculated by this simplified method can be considered as probabilistic safety factors. The probability of failure as a function of the safety factor against collapse is estimated by means of the reliability index β calculated for representative combinations of the coefficients of variation (COV). The effect of the failure probability on the safety factor against fracture in terms of the fracture toughness and the failure assessment diagram for representative combinations of the values COV is analyzed. This approach will allow avoiding more complex and costly probabilistic analysis of failure for fracture based on the fracture toughness distribution.

Failure assessment of pressure vessels made of plain carbon steel by using modified inherent flaw model in DL based industry optimization intelligent processing

Abstract Critical engineering components in the process of designing have employed fail-safe approaches and damage tolerance in industry optimization intelligent processing. Different types of geometrical intricacies were occurring in designing of pressure vessels while considering the parameters. The failures may be due to manufacturing defects or pre-existing flaws present in the cylinder. Even in the probabilistic method of determining failure the structural integrity of the components changes on compressive and tensile loadings. The failures cause the pressure vessels in not adopting its design for the requirement. Normally Inherent Flaw Models (IFM) was used to design wide tensile specimens effectively since the fundamental aspects of material failure can be assessed. DL based Inherent flaw models (IFM) was utilized to generate Failure Assessment Diagrams (FAD) and also to estimate the notched tensile strength of different cracked geometry. IFM is limited only of the wide through cracked tensile specimens. The present work utilizes modified IFM and by converting the pressure vessel failure design data to that of a wide tensile plate by changing its equivalent crack length obtained from an axial crack present in the cylinder. Failure pressure estimation based on fracture criterion is found to be a good coincidence with the data series. Finite Element Analysis (FEA) procedures were utilized to determine plastic zone size of the plate for better validation of the results. Crack Initiation and subsequent growth were identified from the plastic zone.

Effect of integrating polymeric pipes on the pressure evolution and failure assessment in cast iron branched networks

Pressure response and structural integrity assessment of cast iron and HDPE pipes were investigated in this study. The failure analysis of pipes in a branched network was considered taking into consideration the maximum stress generated by hydraulic transient events and the presence of an initial external semi-elliptical defect. The numerical study was conducted based on a developed non-conventional transient solver that incorporates the viscoelasticity of polymeric pipes. Considering the replacement of pipes located in sensitive zones with high-density polyethylene (HDPE) pipes, the pressure evolution of the network was evaluated for different network states at different locations. The integration of polymeric pipes in the originally cast-iron network provided remarkable pressure wave damping and dispersing in some locations. Since the failure of pipes with a corrosion crater is likely to occur when the admissible stress is reached, a structural integrity analysis was conducted. Failure Assessment Diagram (FAD) using the SINTAP code was considered to calculate the safety factors for cast iron pipes with different metallic-plastic combinations, whereas a semi-empirical model that calculates the J-integral is used for HDPE pipes. The results of this study conclude that high attention should be paid when a longitudinal crack is located in the pipelines. It may be possible for integrated polymeric pipes to alleviate the risk of failure in the network. However, under large crack geometries, additional measures should be taken.

Unified constraint-based FAD assessments for ductile fracture in cracked pipes

In this work, the unified constraint-based FAD (failure assessment diagram) assessment methodology has been investigated and developed for ductile fracture in cracked pipes. Two unified constraint parameters Ap and Ad are utilized to incorporate both in-plane and out-of-plane constraints. The results show that the FAD approaches based on two-parameter K-Ap and K-Ad can predict similar fracture initiation loads. With decreasing crack length and depth, the constraint level decreases and fracture toughness increases, thus the predicted fracture load increases. For some shallow and short pipe cracks with lower constraints, the unified constraint-based FAD assessment can reduce conservatism degree of the conventional FAD assessment. For some deep and long cracks, their constraint levels may be higher than the standard plane strain specimen, and the application of the unified constraint-based FAD assessment approaches may eliminate non-conservative assessment in the conventional FAD approach. The unified constraint-based FAD approaches and CDF (crack driving farce) approaches using parameters Ap and Ad can produce similar fracture prediction results, thus they are basically equivalent.

The Determination of the Limit Load Solutions for the New Pipe-Ring Specimen Using Finite Element Modeling

To estimate the acceptable size of cracks and predict the loading limit of the pipeline or its resistance to the initiation and crack growth by following the structural integrity, the fracture toughness and limit load solutions are required. Standard fracture toughness testing of thin-walled pipelines is often difficult to perform in order to complete standard requirements. To find an alternative technique for the measurement of the fracture toughness of the already delivered pipeline segment, the new pipe-ring specimen has been proposed; however, the limit load solutions have not been investigated yet. The limit load depends on the geometry of the specimen and loading mode. The ligament yielding of pipe-ring specimens containing axial cracks through the thickness under combined loads was calculated by the finite element method. This paper provides limit load solutions of several different pipe-ring geometries containing two diametric symmetrical cracks with the same depth ratio in a range of 0.45 ≤ a/W ≤ 0.55. The limit load (LL) solutions calculated by numerical analysis are shown as a function of the full ring section’s size and the corresponding crack aspect ratio for determining the normalized load. These can potentially construct the failure assessment diagram to estimate the crack acceptance in a part of the pipe.

The synergistic effects of hydrogen embrittlement and transient gas flow conditions on integrity assessment of a precracked steel pipeline

We are reporting in this study the hydrogen permeation in the lattice structure of a steel pipeline designed for natural gas transportation by investigating the influence of blending gaseous hydrogen into natural gas flow and resulted internal pressure values on the structural integrity of cracked pipes. The presence of cracks may provoke pipeline failure and hydrogen leakage. The auto-ignition of hydrogen leaks, although been small, leads to a flame difficult to be seen. The latter makes such a phenomenon extremely dangerous as explosions became very likely to happen. In this paper, a reliable method is presented that can be used to predict the acceptable defect in order to reduce risks caused by pipe failure due to hydrogen embrittlement. The presented model takes into account the synergistic effects of transient gas flow conditions in pipelines and hydrogen embrittlement of steel material due to pressurized hydrogen gas permeation. It is found that blending hydrogen gas into natural gas pipelines increases the internal load on the pipeline walls due to overpressure values that may be reached in a transient gas flow regime. Also, the interaction between transient hydrogen gas flow and embrittlement of API 5L X52 steel pipeline was investigated using Failure Assessment Diagram (FAD) and the results have shown that transient flow enhances pipeline failure due to hydrogen permeation. It was shown that hydrogen embrittlement of steel pipelines in contact with the hydrogen environment, together with the transient gas flow and significantly increased transient pressure values, also increases the probability of failure of a cracked pipeline. Such a situation threatens the integrity of high stress pipelines, especially under the real working conditions of hydrogen gas transportation.

Engineering critical assessment of steel shell structure elements welded joints under high cycle fatigue

Existing flaws in critical parts of structural elements may lead to failures of subassemblies and, in case of lack of redundancy, even to the entire structure collapse. Considering the case of cyclic loading – wind load (including extreme phenomena as storms or blizzards), in which some of these type of structures have failed/collapsed, the post factum assessment, indicated the principal damage causes, i.e. existing welding flaws in combination with corrosion (surface flaws). The paper presents the fitness-for-purpose analysis, i.e. Engineering Critical Assessment, applied during design, in order to assist in the choice of welding procedure and/or inspection techniques. The evaluation is made with BS7910/2013 standard using level 2 failure assessment diagrams for possible discovered flaws, considering a real case assessment – a new designed steel monopole type antenna tower structure.

Analysis of Failure Emanating from a Notch

Notch-like defect assessment cannot be done by using classical fracture mechanics (mechanics of cracks). In order to prevent overconservatism, the concepts of notch fracture mechanics, such as notch stress intensity factor or notch energy integral Jρ are generally used for this purpose. A local stress criterion, named volumetric method, defines effective stress. It is used for more advanced assessment when introducing geometry and constraint effects. These effects are taken into account for notch fracture toughness and constraint a modified notch failure-assessment diagram.

The effect of crack-tip constraint in some problems of fracture mechanics

The effect of in-plane and out-of-plane crack-tip constraint parameters, namely, the non-singular T-stress components and the parameter A, in some problems of elastic and elastic-plastic fracture mechanics is discussed. The three-dimensional crack tip stresses including the non-singular T-stress components are employed to describe the crack tip plastic zone, the failure assessment diagrams and the constraint-dependent fracture toughness for a solid with a finite notch/crack under mode I loading and different angles of surface crack growth under conditions of rolling-sliding. Theoretical and numerical aspects of the two-parameter J-A approach in elastic-plastic fracture mechanics are presented in connection with crack-tip constraint. It is shown that the parameter A can be introduced in a fracture criterion as a constraint parameter.

A comparison of failure assessment diagram options for Inconel 690 and Incoloy 800 nuclear steam generators tubes

In the context of structural integrity assessment of cracked Steam Generator Tubes (SGTs), the Failure Assessment Diagram (FAD) has been proposed as a modern tool able to reduce the conservativeness of traditional plastic limit load analyses. As different FAD failure curves are available in the literature, this paper presents a comparison between the so-called Options 1, 2 and 3 FAD curves derived for Inconel 690 and Incoloy 800 nuclear SGTs. Detailed Option 3 curves are obtained from elastoplastic finite element analyses and experimental data from fracture tests. Material-specific Option 2 curves are drawn from stress vs. strain data of uniaxial tests of SGTs. It is shown that the simplest generic Option 1 curve is conservative but appropriate for structural integrity assessments of cracked nuclear SGTs in operation. Alternatively, Option 3 FAD curves must be used when an excessive conservatism shall be avoided.

Study on FAD curves of steel pipes based on stress-strain curves

The failure assessment diagram (FAD) method is one of the mainstream methods for assessment of defects in steel pipes and girth welds. In this paper, the FAD curves based on Ramberg-Osgood model and tensile test requirements for steel pipelines from X52 to X100 are established with an accuracy of less than 3%. The results show that the generalized FAD curve is more conservative in evaluating high-grade defective pipelines than in evaluating low-grade pipelines. Based on the tensile test data of DIC technology, the FAD evaluation models of the weld, base metal and the HAZ are established, and the critical crack lengths are evaluated respectively combining with CVN impact test data. R6 Option 1 and BS 7910 Option 1 are found to be conservative in the same regional assessment, ranging from 20.83% to 31.09%. This study can provide convenience and reference for defect assessment of pipeline and girth weld.

Experimental failure assessment of high tensile stiffened plates with openings

A series of compressive tests of high tensile steel stiffened plates with multiple openings have been conducted. Different degrees of openings represented by different sets of openings are considered, and their effect on the ultimate carrying capacity is explored. The results are discussed, and several conclusions are stated. Based on the experimental results, several relationships predicting the ultimate axial capacity of high tensile steel stiffened plates with multiple openings as a function of remaining volume and degree of openings are developed. Also, a failure assessment diagram of stiffened plates with openings is defined. Finally, a comparison with other published experimental results for deteriorated mild steel plates with multiple opening is performed.

Damage tolerant analysis of corner cracks in small nozzle process penetration of the CAREM-25 reactor pressure vessel

This paper presents a damage tolerant approach assessment of corner cracks in small nozzle process penetrations in the pressure vessel of the small modular reactor CAREM-25 subjected to mechanical and thermal shock loadings during certain operation events. Fracture mechanics analyses were performed following ASME XI guidelines considering a representative geometry and specific loading conditions. Stress distributions in the uncracked component were calculated by finite element techniques, while fracture driving forces were estimated using closed form solutions. Maximum allowable corner crack sizes were calculated using linear elastic fracture mechanics and failure assessment diagram (FAD) approaches. The onset of stable crack growth and crack instability were considered as failure conditions. Additionally, fatigue crack growth estimations during the service life were performed integrating a Paris law up to the maximum allowable crack sizes. The results indicate that it seems unlikely that pre-existing cracks reach allowable sizes during the reactor's service life.

Out-of-plane constraint loss in three point bend specimens with notches

This paper presents an experimental and numerical study of the effect of specimen thickness on the effective notch toughness Kmatρ for cleavage fracture measured using Single Edge Notch Bend (SENB) specimens containing a U-notch instead of a fatigue pre-crack. These specimens are typically used to measure a material's effective notch toughness Kmatρ and to assess failure of a structure containing a non-sharp defect using the Notch Failure Assessment Diagram (NFAD). Both the experimental data and the Finite Element (FE) failure predictions show a significant influence of specimen thickness on Kmatρ, over and above the microstructural weakest link effect arising from differences in the volume of the plastic zone. Kmatρ is a function of not only the in-plane effect of the notch radius, but also an out-of-plane constraint loss which itself is enhanced by the presence of the notch radius. Significant out-of-plane constraint loss occurred for notched specimens with a ratio of thickness B to width W of 0.5, a geometry that if pre-cracked would have met the minimum thickness requirement mandated by ASTM E1820. Doubling the thickness to B/W = 1.0 was sufficient to eliminate the out-of-plane constraint loss. The use of experimentally measured Kmatρ values in an NFAD assessment of a structure may therefore be non-conservative if B/W<1.0.

Fitness-for-service assessment and failure analysis of AISI 304 demineralized-water (DM) pipeline weld crack

Fitness-for-service assessment (FFS) is employed in oil and gas refiners to assess the structural integrity of an in-service pressure vessels and storage tanks that may contain a flaw or damage. API 579/ASME FFS-1 standard provides guidance for conducting FFS assessments using methodologies specifically prepared for pressurized equipment. In the present investigation, FFS assessment (level 3, based on the Failure Assessment Diagram (FAD)) carried out on a 304 demineralized water stainless steel pipeline containing a crack at weldment eleven months before the failure is explained in detail. Despite the FFS assessment and acceptance of the pipeline, failure occurred eleven months after the FFS assessment. The distribution of the residual thermal stress introduced during welding was estimated by means of Finite Element Analysis (FEA) using Abaqus software. Further investigation revealed that the operating condition of the DM pipeline deviated from the date of FFS assessment and the failed pipeline had been under vibration. Defects such as inclusions and porosities and irregular shape of weld bead were observed in weld joint showing lack of proper quality control (QC) during welding and installation of the pipeline. The heat affected zone (HAZ) region was found to be sensitized (perhaps due to the improper welding procedure during installation) and severe precipitation of chromium carbides was observed on the grain boundaries. Further investigation showed the transgranular nature of the crack propagated perpendicular to the weld metal as a result of the dynamic loading caused by steam hammering effect leading to the vibration-induced fatigue. The Scanning Electron Microscopy (SEM) of the fracture surfaces showed that the fracture surfaces consist mostly of fatigue striations and quasi-cleavage fracture.

Constraint modified time dependent failure assessment diagram (TDFAD) based on C(t)-A2(t) theory for creep crack

Due to the potential influence of constraint effect on failure assessment, a link is needed to bridge the constraint effect characterized by higher order term asymptotic solution and time dependent failure assessment diagram (TDFAD). In this paper, a modification method for creep toughness incorporating C(t)-A2(t) theory is proposed based on Nikbin-Smith-Webster model. The relation between Q-term and A2(t)-term is also presented for creep crack. Results show that the creep toughness can be influenced by constraint parameter remarkably. A modified TDFAD is proposed based on C(t)-A2(t) solution. Based on the creep data of 316H stainless steel and various compact tension specimens, the modified TDFAD curves considering constraint effect characterized with A2(t)-term is presented and discussed, which indicates the important role of constraint effect on the assessment boundary of TDFAD. The research given in this paper can provide an alternative method in which the constraint effect can be included in TDFAD under high temperature.